Wednesday, May 13, 2015

Precipitation Skewness

The Fairbanks area received its first measurable precipitation in almost a month over the weekend: 0.17" of rain at the airport, 0.32" at Keystone Ridge.  Climatologically speaking, the frequency of rainfall will begin to pick up in the next couple of weeks as the season of summer showers starts to get under way.

One aspect of the precipitation climatology that I don't think I've examined before is the seasonal change in skewness of precipitation amounts.  For monthly precipitation totals, this is a simple calculation and produces the following result:

We see that monthly precipitation skewness is high in the winter but relatively low in the warm season; and there is a large change between April and May.  This means that May precipitation totals are distributed relatively evenly above and below the median, whereas April precipitation has a highly skewed distribution.  Of course, April is much drier than May on average, and large precipitation skewness tends to accompany dry climates.  As an example of the April skewness, consider that the April long-term median is 0.16", but the mean is nearly twice as much, at 0.29".

The high skewness value in January for 1930-2014 is not an error: this is caused solely by the extreme precipitation total of 6.71" in January 1937.  Removing the 1930's from the calculation gives a smoother result in terms of the month-to-month variability, but we can speculate that these results underestimate the "true" very-long-term skewness because we are not adequately sampling the most extreme events.

While I'm on the subject of precipitation extremes, it's interesting to look at changes in the frequency of monthly extremes over time in Fairbanks.  The chart below shows the number of months that fell in the top 10% of the 1930-2014 distribution, by month, for each decade since 1930, and I've estimated the numbers for 2010-2019 by doubling the 2010-2014 total.

Interestingly there is a suggestion of a downward trend in the frequency of large monthly precipitation amounts, at least until this decade.  The downward trend in the cold season is more prominent and is right at the 95% level of statistical significance if we exclude the current decade.  Obviously, however, the past few years have not continued this trend towards fewer wet extremes.

Does this result imply that Fairbanks has become drier over time?  Yes; based on the 1930-2014 history, the linear trend of annual totals is towards drier conditions at a rate of 0.16" or 1.5% per decade; but this is not statistically significant.  Among the individual months, there is considerable variability in the trends; the only significant trend is in August, which has become drier at the 99% level (the linear trend is 6% per decade).

For related reading on this blog, I'll refer to Brian's post from last August:

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